1. Field of the Invention
The present invention relates to an ionic liquid and a power storage device including the ionic liquid.
2. Description of the Related Art
Owing to the increase in demand for portable electronic devices such as a mobile phone and a laptop personal computer and the development of electric vehicles (EVs), the demand for power storage devices such as an electric double layer capacitor, a lithium-ion secondary battery, and a lithium-ion capacitor has been significantly increasing in recent years. For the power storage devices, high capacity, high performance, safety in various operating environments, and the like are required.
To satisfy the above requirements, electrolyte solutions for power storage devices have been actively developed. Cyclic carbonates are used for electrolyte solutions for power storage devices. In particular, ethylene carbonate is often used because of its high dielectric constant and high ionic conductivity.
However, not only an ethylene carbonate but also many organic solvents have volatility and a low flash point. For this reason, in the case where an organic solvent is used for an electrolyte solution of a power storage device, the temperature inside a lithium-ion secondary battery might rise due to a short circuit, overcharge, or the like and the lithium-ion secondary battery might burst or catch fire.
In consideration of the risks, use of an ionic liquid, which is nonvolatile and flame-retardant, for an electrolyte solution of a power storage device has been studied. An ionic liquid is also referred to as ambient temperature molten salt, which is a salt formed by combination of cations and anions. Examples of the ionic liquid include an ionic liquid including quaternary ammonium-based cations and an ionic liquid including imidazolium-based cations (see Patent Document 1 and Non-Patent Document 1).